Revert 179125 "Drop the 'WebURLRequest::TargetType' enum."

Believe changes related to this introduced flakiness again in the
browser. See Issue 390497.

> Drop the 'WebURLRequest::TargetType' enum.
> 
> We no longer use this enum in Chromium; everything has been ported over
> to using '::RequestContext' and '::FrameType', so we can safely remove
> both the enum and the translation machinery in '::setTargetType'.
> 
> BUG=390497
> 
> Review URL: https://codereview.chromium.org/417483002

TBR=mkwst@chromium.org

Review URL: https://codereview.chromium.org/422343003

git-svn-id: svn://svn.chromium.org/blink/trunk@179162 bbb929c8-8fbe-4397-9dbb-9b2b20218538